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Hii

Can anyone tell me in what scenario an AP will send NULL data frame to the station ?I got this frame while playing with analyzer today.

With thanks

Vinay Saini

http://www.wi-fiplanet.com/tutorials/article.php/3442991

Hope this helps.

Null Data Frames

Some wireless LAN vendors use null data frames, which contain an empty frame body, to carry special control information to another station. For example, wireless clients commonly use a null data frame sent to the access point to indicate a change in sleep state by setting the power management bit in the frame control field appropriately. This most often occurs after a wireless client implementing power save mode has been a wake receiving buffered frames from the access point. The null data frame tells the access point to start buffering frames again for that client station because the client is going back to sleep.

Another use of null data frames is part of active scanning. The client station sends a null data frame to the access point to indicate sleep state prior to performing active scanning, which is the process of looking for access points on different channels for the purpose of possible roaming. The access point then buffers frames for the client station while the client scans other RF channels. While on another channel, the client cant receive frames from the access point.

Once the client station comes back to the associated access point channel, the client sends another null data frame to the access point with the power management bit reset to indicate that the client is ready to receive frames again. This maneuver somewhat fools the access point to think that client is in sleep mode; however, it works very well to reduce frame retransmissions while the client is busy scanning other channels.

Thanx for response compughter

But mine question is for Null frames from AP to Station.

Ur explanation is correct for the Null frames from station to AP.

Plz explain me Null Frame from AP to station.

Regards

Vinay saini

Is QoS enable??? They are QoS-NULL frames that are sent by the AP. Like an AP in PCF mode.

Thk,
P.

No QOS is not enable , its simple DCF based netwok .

can you post your frame capture here?

Hii Rdioman

The scenario is like this :

I have an 802.11g Ap set to work in B only mode, i have connected mine B clent ,after authentication and association frames , the Ap kept on sending the NULL data frames to station at regular interval of bout 1 minute.

Thanx

Vinay

hii Radiomen and all

I have again get the same frame , Ap is sending NULL to station,
In this scenario , Ap is configured for B only mode and station associated is in power save mode, i can see the null frames from station to Ap with powersave bit on and off, but sometimes Null frame from that Ap to this station also comes.
Decode of this frame gives this info:


ver:0
Type:10
Subtype:0100

Frame control flags ::
0 order
0 prot. bit
0 more data
0 ---
0 ---
0 ---
1 exit from distribution system
0 not from DS

Duration : 314
Destination: 00:0f:3d:fa:59:64
BSSID: 00:0f:66:19:6e:9a
source: 00:0f:66:19:6e:9a
seqno: 2625
frag no: 0

this is correct frame with no CRC error.

Thanx

Vinay Saini

hii Radiomen and all

I have again get the same frame , Ap is sending NULL to station,
In this scenario , Ap is configured for B only mode and station associated is in power save mode, i can see the null frames from station to Ap with powersave bit on and off, but sometimes Null frame from that Ap to this station also comes.
Decode of this frame gives this info:


ver:0
Type:10
Subtype:0100

Frame control flags ::
0 order
0 prot. bit
0 more data
0 ---
0 ---
0 ---
1 exit from distribution system
0 not from DS

Duration : 314
Destination: 00:0f:3d:fa:59:64
BSSID: 00:0f:66:19:6e:9a
source: 00:0f:66:19:6e:9a
seqno: 2625
frag no: 0

this is correct frame with no CRC error.

Thanx

Vinay Saini

Hey Vinay Saini

What type of AP are you using? That?¡é?€??s a 10-0100 is correct, a data (null) frame.
Any interesting frames before that one? I never have seen that one before; Ap?¡é?€??s never enter power-saving mode and do not transmit Null Frames.

Are you always using the same analyzer?

thanks,
P.